Cleaning of magnesium base alloy castings



United States Patent CLEANING 0F MAGNESIUM BASE ALLOY CASTINGS No Drawing. Application May 6, 1954, Serial No. 428,120

Claims priority, application Great Britain July 4, 195 1 2 Claims. (Cl. 204-141) This invention relates to the cleaning of castings of magnesium base alloys and is a continuation in part of application No. 294,483 filed 19th June 1952, now abandoned.

Castings of such alloys are almost invariably contaminated with the ingredients of the moulding composition particularly .core compositions, which become attached to or embedded in the surface of the casting. This contamination is difficult to remove and consequently it has been common practice to clean the casting by sand blasting or shot blasting. However, although this procedure is effective in cleaning the casting from the moulding composition contaminants, the resultant casting is found to have considerably less resistance to corrosive action than castings which are cleaned by strong chemical agents or by machining, both of which are in many cases undesirable for various reasons.

I have now ascertained that such castings can be cleaned by a comparatively simple process without deleterious effect on the resistance of the casting to corrosion and substantially without dimensional change such as could be brought about by acid pickling for example, or by machining or scurfing, thus avoiding the obliteration of foundry code marks which frequently happens at present.

According to the present invention the casting is subjected with its moulding contaminants thereon to anodic treatment whilst immersed in an aqueous solution of ammonium fluoride (NH4F or NH4F.HF) in quantity at least two percent ammonium fluoride by weight of the water, and continuing the treatment until current flow is substantially stopped automatically.

The solution may contain ammonium fluoride from 2 percent up to the quantity representing the limit of solubility together with ammonia in quantity corresponding to 5-15 of aqueous ammonia solution of .880 specific gravity.

The casting to be treated is made the anode in an electrical circuit supplied with D. C. or A. C. current. As

the treatment proceeds the voltage increases and the cur- The casting will then be found to be free from the surface contaminants referred to.

The time of treatment depends on various factors such as current density and the degree of contamination of the casting, and may take from half a minute or less for fairly clean castings and up to thirty minutes or more for badly contaminated castings. The time of treatment will however usually be at least fifteen minutes.

As a particular example of the process according to the invention the following may be mentioned:

The electrolyte was prepared by adding to a bath of water 10% by weight of ammonium bi-fluoride and 10% of an aqueous solution of ammonia of .880 specific gravity. The castings were suspended in this electrolyte by means of magnesium clamps which were made the anode in an electrical circuit which included a source of D. C. current supply of volts. At the commencement of the treatment the voltage fell to 5 and the current density was about 5 amps. per square foot of casting. After ten minutes treatment the cur-rent density fell to a few microamps and the voltage rose to 80. The current was then switched 01f and the castings removed and rinsed in running water and were found to be completely free from burnt-on and embedded mould and core compositions.

I am aware that it has heretofore been proposed to submit articles of magnesium base alloys to anodic treatment in fluoride-containing aqueous solutions for the purpose of producing protective layers thereon. These treatments however have not proved altogether satisfactory and have not found any general application inindustry. Moreover for all such anodising treatments the practice has always been to clean the articles before anodising and it has not been previously known that anodising in such solutions would efficiently remove mould and core contaminants burnt on or embedded into the surface of the castings.

I claim:

1. A method of cleaning castings of magnesium base alloys having mould contaminants in the surface thereof which comprises removing said mould contaminants thereon by subjecting the said casting to anodic treatment whilst immersed in an aqueous solution consisting of ammonium fluoride, said ammonium fluoride being selected from the group of fluorides consisting of NH4F and NH4F.HF, said aqueous solution containing in quantity at least two percent ammonium fluoride by weight of the water.

2. A method as claimed in claim 1 wherein the solution also contains ammonia of .880 specific gravity in quantity corresponding to 5 to 15 percent by weight of the water.

References Cited in the file of this patent UNITED STATES PATENTS 7 1,574,289 Keeler Feb. 23, 1926 2,468,006 Webster Apr. 19, 1949 FOREIGN PATENTS 840,385 France Jan. 16, 1939 660,409 Germany May 24, 1938 294,237 Great Britain Sept. 12, 1929 

1. A METHOD OF CLEANING CASTINGS OF MAGNESIUM BASE ALLOYS HAVING MOULD CONTAMINANTS IN THE SURFACE THEREOF WHICH COMPRISES REMOVING SAID MOULD CONTAMINANTS THEREON BY SUBJECTING THE SAID CASTING TO ANODIC TREATMENT WHILST IMMERSED IN AN AQUEOUS SOLUTION CONSISTING OF AMMONIUM FLUORIDE, SAID AMMONIUM FLUORIDE BEING SELECTED FROM THE GROUP OF FLUORIDES CONSISTING OF NH4F AND NH4F.HF, SAID AQUEOUS SOLUTION CONTAINING IN QUANTITY AT LEAST TWO PERCENT AMMONIUM FLUORIDE BY WEIGHT OF THE WATER. 